The present invention relates to a multi-layer product, the first layer being a UV-cured protective layer which contains SiO2 nanoparticles, and the second layer containing a thermoplastic substrate. In addition, the invention relates to the composition of the UV-curable first layer, a process for the production of the multi-layer products and products, such as e.g. glazing products, which contain the named multi-layer products.
C. Roscher in Pitture e Vemici—European Coatings 2004, 20, 7-10, discloses UV-curable organic coating systems containing nanoparticles of silicon dioxide as a coating system which, in comparison to corresponding filler-free coating systems, have a clearly improved scratch resistance and abrasion resistance.
Moldings made of polycarbonate have been known for some time. Polycarbonate, however, has the disadvantage that it is not itself inherently UV-stable. The sensitivity curve of bisphenol A polycarbonate has the highest sensitivity between 320 nm and 330 nm. Below 300 nm, no solar radiation reaches the earth, and above 350 nm this polycarbonate is so insensitive that no further yellowing takes place.
Thus, for a durable coating of a UV-sensitive plastic substrate such as polycarbonate, to produce a multi-layer product also suitable for long-lasting external application, efficient UV protection is required in the first layer.
Typical UV stabilisers used in coatings, are UV absorbers such as 2-hydroxybenzophenones, 2-(2-hydroxyphenyl)benzotriazoles, 2-(2-hydroxyphenyl)-1,3,5-triazines, 2-cyanacrylates and oxalanilides, and radical scavengers of the HALS (hindered amine light stabilizer) type. These additional coating components, with a UV-curing binder, affect the radical crosslinking reaction started by UV light, by competing with the photoinitiator for the UV light or by intercepting the initiator radicals or secondary radicals formed.
In the following, the prior art is combined with multi-layer products, with a first layer consisting of a matrix filled with organic nanoparticles formed by UV curing and containing a UV absorber.
EP-A 0 424 645 discloses a UV radiation-curable coating based on acrylates and colloidal silicon dioxide, in which UV absorbers, specifically a benzophenone type, a cyanacrylate type and a benzotriazole type, and radical scavengers of the HALS type, are mentioned as possible additives. When UV light is used for radiation curing, there is the problem of a hindering of curing as a function of the quantity of UV absorber. Regarding the photoinitiator for curing, according to EP-A 0 424 645 there are no restrictions; 2-hydroxy-2-methyl-1-phenylpropan-1-one (Danocure® 1173 from Ciba Speciality Chemicals) and 2,2-dimethoxy-1,2-diphenylethan-1-one (Irgacure® 651 from Ciba Speciality Chemicals) are specifically mentioned.
EP-A 0 576 247 discloses a coating curable by means of UV radiation based on colloidal silicon oxide, silyl acrylate, acrylate, 2,4,6-trimethylbenzoyldiphenyl phosphine oxide (Lucirin TPO from BASF AG) as photoinitiator and UV absorbers. Sterically hindered amines of the HALS type, fluoroacrylate and alkyl acrylate can optionally be used as additives. Three benzophenone types and two benzotriazole types are named with Cyasorb® UV-416, Cyasorb® UV-531, Cyasorb® UV-5411, Tinuvin® 328 and Uvinul® 400 mentioned as suitable as UV absorbers.
U.S. Pat. No. 5,468,789 discloses a coating curable by means of UV radiation based on colloidal silicon oxide, alkoxysilyl acrylate, acrylate monomer and a special gel-forming inhibitor, wherein optionally UV absorbers such as resorcinol monobenzoate and 2-methylresorcinol dibenzoate can be included.
To achieve adequate protective function for the second layer, the first layer should filter out or absorb the UV light that is harmful for the second layer as much as possible. In the case of polycarbonate as the second layer, therefore, UV light with a wave length of 300 to 340 nm should be filtered out or absorbed from the first layer. This requires both a corresponding quantity of a UV absorber with a high absorption coefficient in the first layer and an adequately thick first layer. This high UV filter action of the UV absorber competes in the curing of the first layer initiated by UV light with the photoinitiator for the light required for the formation of radicals. The additional use of radical scavengers such as HALS as light stabilisers is also difficult because they can intercept the radicals required for the radical start of crosslinking and that are formed from the initiator by UV radiation.
An object of the present invention is to provide multi-layer products with a UV-sensitive substrate such as polycarbonate as the second layer and a UV-curing scratch-resistant and abrasion-resistant protective layer as the first layer. The multi-layer products are distinguished by effective UV protection and high abrasion resistance.
This object is surprisingly achieved by a coating formulation containing at least one special photoinitiator selected from the group consisting of acylphosphine oxide derivatives and α-aminoalkylphenone derivatives which, after application and curing, forms the first layer of the multi-layer product. These special photoinitiators produce, optionally in combination with other photoinitiators, the effective UV-initiated crosslinking necessary for the high abrasion resistance and scratch-resistance which is not impaired by the presence of the UV stabilisers such as triazine derivatives and optionally radical scavengers of the HALS class. The abrasion resistance of the multi-layer products according to the invention with implemented UV protection is on a comparable level to corresponding multi-layer products with siloxane-based coating systems.